The present invention relates to thermal printing devices and, more particularly, to a method for controlling the take-up ribbon tension of the thermal ribbon cassette.
In the development of a novel thermal printing postage meter consideration was given to utilizing a replaceable thermal transfer ribbon cassette. In utilizing a thermal transfer ribbon cassette in combination with the novel thermal printing postage meter, it is considered advantageous to utilize a postage meter configuration whereby a driven platen would be singularly responsible for displacement the print media, i.e., envelope, and the thermal transfer ribbon in order to insure synchronized printing. In order to further insure print quality, it is considered advantageous to maintain a constant print ribbon tension during the printing cycle.
Conventionally, tension control is provided by a clutched take-up system. However, due to the constantly changing radius of the take-up spool, a fixed input clutch produces a high web tension in the beginning of the cassette tape and a low web tension at the end of the cassette tape. It is further noted that the ribbon once partly relieved of transfer ink due to the printing process is difficult to wind uniformly on the take-up spool. After the printing process that portion of the spent transfer ribbon is severely weakened and distorted due to the printing process, too much web tension can cause induced wrinkles in the printing area as well as uneven winding on the take-up spool resulting in an overly large take-up spool diameter which may also be the result of too low web tension.
For postage meter application, it is a fur advantage to utilize a compact ribbon in order to maximum use of the web ink area which requires overlapping of the supply side and take-up side radii. As a result, it is important that the take-up spool wind properly to avoid collision between the supply side radius and the take-up side radius.